A critical function of Emergency Core Cooling Systems (ECCS) and other recirculation systems of nuclear power plants is to move fluids quickly and in large volumes to critical areas of the nuclear power plant in the event of accidents and emergencies. Integral to this critical function is the ability of strainers, filters, screens and other such devices associated with the systems to remove solids from the moving fluids while at the same time maintaining a sufficiently large volume of fluid flow.
Suction strainers are used in suppression pools of Boiling Water Reactor (BWR) nuclear power plants to remove solids from the fluid stored in the suppression pools when the fluid is drawn into an Emergency Core Cooling System (ECCS) or other recirculation system. The goal is to have strained fluid substantially free from particulate matter, thereby minimizing pump degradation.
In the United States and other countries, there are generally three different types of BWR nuclear power plants. The most common of these is the Mark I, followed by the Mark II and finally the Mark III. Each type of BWR nuclear power plant has a different suppression pool design. Generally speaking, the Mark I has a toroidal-shaped suppression pool, the Mark II has a simple circular tank, and the Mark III can best be described as a moat around the power plant. The differences in suppression pool design, as well as other plant design differences, have made the construction of a universally adaptable suction strainer unfeasible. Moreover, retrofitting upgraded suction strainers in existing BWR nuclear power plants is an extremely difficult task.
A universal goal in the nuclear power plant field has been to increase the effective surface area of suction strainers so that the required volumetric flow rate of water can be delivered to the reactor following a loss of coolant accident (LOCA). A LOCA can result when a high pressure pipe ruptures with such great force that large quantities of debris from thermal insulation, coatings, concrete, and other sources can wash into the suppression pool, thereby clogging the suction strainer(s). As a result, the volumetric flow rate of cooling water delivered to the reactor can be drastically reduced which, in turn, can lead to reactor core overheating. The thrust of recent advancements in the suction strainer art has been directed toward designing suction strainers that can adequately filter such debris from the suppression pool fluid without becoming clogged (i.e., without leading to a reduction in ECCS pump volumetric flow rate). Following a LOCA, it is critical that the ECCS pumps can operate undegraded for extended periods of time. To achieve this result, large quantities of fluid, free from solids and other particulate matter, must reach the pumps. Recent advances have yielded suction strainers that can adequately filter debris from the fluid to limit pump degradation, but the goal of increasing the surface area of suction strainers so that greater volumes of water can be delivered to the reactor has been more difficult to achieve in some BWR plants. This is due to the second effect of a LOCA.
The second effect of a LOCA in a BWR plant is the generation of post-LOCA hydrodynamic forces. Following a LOCA, high pressure steam is expelled from the reactor through structures known as downcomers which extend into the suppression pool. The resulting hydrodynamic forces created within the suppression pool place extreme loads upon any protruding structure within the pool, including suction strainers. While one function of the suppression pool is to condense this steam and thereby quickly dissipate these high pressures, significant hydrodynamic forces are still applied to the structural features and protrusions within the pool. In general, the greater the length and diameter of the suction strainer, the greater the resulting load on the strainer. For this reason, while it is easy to design a suction strainer having an increased surface area by increasing the overall length and diameter of the suction strainer, it is difficult to support such a strainer and, in many cases, to install such a strainer.
Heretofore, various suction strainers have been employed for the general objective of filtering solids from the fluid stored within a suppression pool of a BWR nuclear power plant. One such suction strainer design is the cantilevered suction strainer. Such suction strainers typically extend into the suppression pool, are connected to the ECCS suction pipe at one of its ends, and simply cantilever off that suction pipe end. That is the only means of support. Due to the extreme loads which result from post-LOCA hydrodynamic forces and the limited load carrying capabilities of the ECCS pipe and pipe penetration (that portion of the suppression pool wall adapted to receive the ECCS pipe to place the ECCS pipe in fluid communication with the suppression pool), the overall length and diameter of the cantilevered suction strainer is limited. For a given strainer diameter, if the cantilevered suction strainer is too long, the torque applied to the suction strainer by the post-LOCA hydrodynamic forces can damage the suction pipe to which it is attached and/or the penetration through the suppression pool wall.
An advancement in cantilevered suction strainer design is disclosed in U.S. Pat. No. 5,696,801. The suction strainer disclosed in this Application includes a filtering surface defined by a filtering structure that is attached to and built around an internal core tube. Reinforcing structural members extend outward radially from the internal core tube and provide support for the filtering structure. The external filtering structure is formed from a plurality of perforated plate assemblies positioned adjacent one another along the length of the core tube. The plate assemblies extend radially at alternating distances from the internal core tube thereby forming alternating protrusions and troughs. In this way, the surface area of the filtering surface is increased without increasing the overall length of the filtering structure. Openings in the internal core tube allow water from the suppression pool to be drawn through the filtering structure through perforations in the filtering surface. This configuration promotes controlled fluid in-flow along the suction strainer and substantially precludes the establishment of non-uniform localized entrance velocities through the filtering surface. The unique configuration of the external filtering structure enlarges the filtering surface area while minimizing the projected area of the suction strainer. Thus, more water can be drawn through this cantilevered suction strainer without increasing the overall distance this suction strainer extends into the suppression pool. While the overall filtering surface area of cantilevered suction strainers can now be increased, for a given strainer diameter, such suction strainers are still hampered by length limitations.
Other advancements in the art have been made by Sulzer Thermtec. Sulzer Thermtec has designed an elongated simple cylindrical strainer that appears to use a rib-type cage to support a perforated plate. The perforated plate performs the straining function while the cage provides structural support for the plate. The strainer extends parallel to and along the wall of the suppression pool and is connected at one end to the suction pipe with a 90.degree. tee. There is no internal core tube. In order to withstand the extreme forces in the pool, the strainer is secured to the suppression pool wall at each of its ribs. Legs extending from each rib are apparently bolted or otherwise attached to the walls of the suppression pool. Again, installation can be time consuming and difficult, particularly if the suppression pool cannot be drained and if welding is required for strainer installation. Also, most BWR plants cannot accommodate a strainer diameter larger than 3 or 4 feet.
While the suction strainers described above remove solids from the fluid stored within the suppression pools of BWR nuclear power plants, it appears that neither is capable of handling the LOCA generated debris, being installed within geometrically limited diameters, and being supported adequately.
What is needed, therefore, but seemingly unavailable in the art, is a suction system that can (1) handle the postulated debris quantities, (2) be adequately supported and withstand LOCA generated forces, and (3) be installed without modifying the shell in the suppression pools of BWR nuclear power plants.
Unlike a BWR nuclear power plant, a Pressure Water Reactor (PWR) nuclear power plant does not utilize a suppression pool. Rather, a PWR nuclear power plant, both light water and heavy water types, has a containment area which remains dry until an accident occurs. In conventional PWR nuclear power plants, an accident results in the containment area being partially flooded with water and the ECCS relying on a sump pump to circulate the water through the reactor. Typically, the water is filtered through a structurally protective trash rack and then through a finer debris screen to separate particulate matter from the water passed through the ECCS. The suction strainer of the type utilized in a BWR nuclear power plant is not typically found connected to a PWR's ECCS suction piping. Typically, the volume and rate of fluid (e.g., water) flow recirculating through the ECCS is dependent upon the size of the sump pit as well as the overall size of the inlet orifice and related trash rack and debris screen. Accordingly, the volumes and rates of fluid flow in a prior art PWR nuclear power plant were limited by the structural limitations of these sump structures and fixtures. What is needed, therefore, is a manner of retrofitting PWR nuclear plants to overcome the surface area limitations of configurations already existing and, thereby, maintain rates of fluid flow through the ECCS that is encumbered by LOCA generated debris.